(1) Field of the Invention
The present invention relates to a light-sensitive composition. More particularly, the present invention relates to a light-sensitive composition which can be developed with an alkali aqueous solution, and which is suitable for use in the making of a presensitized plate from which a lithographic printing plate having an increased press life and which has a tough image in highlight areas can be prepared.
(2) Description of the Prior Art
Diazonium compounds are widely used as sensitizers in presensitized printing materials, and the most commonly employed diazonium compounds are diazo resins, typically the condensation products of p-diazodiphenylamine and formaldehyde.
In preparing a light-sensitive layer for use in a presensitized lithographic printing plate, the diazo resin may be used either alone as shown in U.S. Pat. No. 2,714,066 (i.e., in the absence of any binder) or in admixture with a binder as described in Japanese Patent Application (OPI) (the term "OPI" (i.e. Open to Public Inspection)" as used herein means an unexamined published Japanese patent application) No. 30604/1975 corresponding to U.S. Pat. No. 4,275,138. In most of the diazo-sensitized lithographic printing plates manufactured today, a diazonium compound is used together with a polymer binder in order to provide for more increased press life.
Light-sensitive layers which comprise a diazonium compound and a polymer binder may be divided into two types, depending on the type of developer used to remove (i.e., develop) the layer in unexposed areas: (1) the "alkali" developable type of light-sensitive layer which is treated with an aqueous alkali developer, and (2) the "solvent" developable type of light-sensitive layer which is treated with an organic solvent-based developer (see U.S. Pat. No. 4,275,138). The alkali-developer type is increasingly used today, principally for safety and environmental reasons due to problems relating to disposal of the fatigued developer. How well this type of light-sensitive layer performs is determined by the properties of the binder used. The binder may be rendered alkali-developable either by copolymerizing it with a carboxylic acid-containing monomer as described in U.S. Pat. No. 4,275,138, or by reacting the hydroxyl group in polyvinyl alcohol with a cyclic acid anhydride such as phthalic anhydride to introduce the carboxylic acid group into the polymer as shown in U.S. Pat. No. 2,861,058. Whichever method is used, the resulting polymer has only a low level of abrasion resistance because of its structure, and the presensitized plate using this type of carboxylic acid-containing polymer as a binder in the light-sensitive layer provides a lithographic printing plate having a short press life. When polyvinyl acetal is used as the binder, a sturdy film is formed having high abrasion resistance; however, the lithographic printing plate sensitized by a layer containing polyvinyl acetal as the binder is of the organic solvent-developable type described above and cannot be treated with aqueous alkali developers.
It has been proposed that the abrasion-resistant polyvinyl acetal resin be modified to make the resin alkali-soluble. For instance, as described in British Pat. No. 1,396,355 a method in which vinyl acetate and a carboxylic acid-containing monomer are copolymerized and the resulting polymer is saponified and converted to the acetal form is proposed. However, the modified polyvinyl acetal obtained by this method does not exhibit satisfactory abrasion-resistance.
The abrasion-resistance of polyvinyl acetate resin was not impaired by the method consisting of introducing a substituted group having an acid hydrogen atom into remaining hydroxide group in a polyvinyl alcohol resin (herein referred to as "PVA resin") which had previously been converted to the acetal form. Examples of such types of methods include a method as described in British Pat. No. 1370316 in which arylsulfonylurethane is introduced into a PVA resin, a method as described in German Patent Publication (OLS) No. 3404366, European Patent Publication No. 208145 in which an intermolecular acid anhydride of organic polycarboxylic acid is reacted with PVA resin, and a method in which an ingredient containing hydroxyl groups or cyano groups is introduced into PVA resin reacted with the intermolecular acid anhydride. However, a presensitized lithographic printing plate obtained by using this modified polyvinyl acetal resin as a binder has a disadvantage in that an image on the highlight portion peels off when the plate is subjected to strong stress on development.